In my 750 X2 I ran a stock 650 pipe, mariner 650 exhaust, westcoast 650 exhaust, and a coffman 750 exhaust for a short period. The coffman made the best power but I didnt like where that power was made , to high up in the rpm range and it was loud as
. And it was a major pain in the a$$ to install. Superjet pipe, or am 650 pipe will be your best bet. I ran the westcoast 650 exhaust for the majority of the time, easiest to install and made good power. A lot more power than the same setup with a 650 motor as well.
Also, I did what Austin mentioned by measuring the volume of the pipes. The mariner was the largest, then the coffman 750, then the westcoast, but they were all within 5% of each other.